Process for removing methacrylic acid from liquid phase comprising acrylic acid as a main constituent and target product, and methacrylic acid as a secondary component

ABSTRACT

A process for removing methacrylic acid from liquid phase P comprising acrylic acid as a main constituent and target product, and methacrylic acid as a secondary component in which the removal is effected by crystallization, the acrylic acid accumulating in the crystals formed and the methacrylic acid in the remaining mother liquor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for removing methacrylic acidfrom liquid phase comprising acrylic acid as a main constituent andtarget product, and methacrylic acid as a secondary component.

2. Description of the Background

Acrylic acid is an important monomer which finds use as such or in theform of its alkyl esters for obtaining, for example, polymers suitableas adhesives or water-superabsorbent polymers (cf., for example, WO02/055469 and WO 03/078378).

Acrylic acid is prepared on the industrial scale worldwide virtuallyexclusively by the (generally two-stage) heterogeneously catalyzedpartial oxidation process of propylene (cf., for example, EP-A 990 636,U.S. Pat. No. 5,198,578, EP-A 10 15 410, EP-A 14 84 303, EP-A 14 84 308,EP-A 14 84 309 and US-A 2004/0242826). The starting propylene used ispropylene of comparatively high purity (cf. DE-A 101 31 297). It isrelatively inconvenient and costly to obtain such pure crude propylene.It normally originates from crude paraffinic hydrocarbons and generallyincludes various purification stages in order to isolate the propyleneformed in highly pure form (cf. DE-A 35 21 458). These purificationstages generally comprise separations from olefins other than propyleneand from other secondary products other than propylene, including thesecondary components already comprised in the crude paraffinichydrocarbon.

Of particular significance in this context is the separation ofpropylene from its companion propane. Owing to the physical similarityof the two compounds, this removal in particular is capital- andenergy-intensive. Since the predominant amount of the thus obtainedcrude propylene is used in large amounts for subsequent polymerizations(for example to prepare polypropylene), (where the high purity describedis indispensable) and experiences a high addition of value, theaforementioned removals are customary in conjunction with refinerycrackers and steamcrackers in spite of the associated cost andinconvenience, and form the state of the art in industry. The proportionof these crude propylenes flowing into the partial oxidation to acrylicacid is of rather minor importance compared to the demand forpolypropylene, and is a secondary demand stream at raw material priceswhich are still acceptable.

It is characteristic of the preparation of acrylic acid by catalyticpartial oxidation in the gas phase of such comparatively pure propylenethat the acrylic acid, in spite of the available purity of the rawmaterial, is not obtained as such, but rather, especially owing toparallel side reactions, as a constituent of a product gas mixture fromwhich it subsequently has to be removed.

This product gas mixture generally also comprises reactants which havenot been converted fully, if appropriate intermediates which have notbeen converted fully and, for reasons of improved heat transfer and toensure nonexplosive behavior, additionally used inert diluent gas.

In this document, an inert diluent gas shall be understood to be areaction gas constituent which behaves inertly under the conditions ofthe relevant reaction and, each inert reaction gas constituent viewedalone, remains chemically unchanged to an extent of more than 95 mol %,preferably to an extent of more than 97 mol %, or 98 mol % or 99 mol %.

A common feature of substantially all separation processes known in thisregard is that, if appropriate after direct and/or indirect cooling ofthe aforementioned product gas mixture, acrylic acid comprised in theproduct gas mixture is transferred in a basic removal step into thecondensed (especially liquid) phase.

This may be effected, for example, by absorption into a suitable solvent(e.g. water, high-boiling organic solvents, aqueous solutions) and/or bypartial or substantially full condensation (e.g. fractionalcondensation) (on this subject, see the documents cited at the outset,and also the documents EP-A 13 88 533, EP-A 13 88 532, DE-A 102 35 847,EP-A 79 28 67, WO 98/01415, EP-A 10 15 411, EP-A 10 15 410, WO 99/50219,WO 00/53560, WO 02/09839, DE-A 102 35 847, WO 03/041833, DE-A 102 23058, DE-A 102 43 625, DE-A 103 36 386, EP-A 85 41 29, U.S. Pat. No.4,317,926, DE-A 198 37 520, DE-A 196 06 877, DE-A 190 50 1325, DE-A 10247 240, DE-A 197 40 253, EP-A 69 57 36, EP-A 98 22 87, EP-A 10 41 062,EP-A 11 71 46, DE-A 43 08 087, DE-A 43 35 172, DE-A 44 36 243, DE-A 19924 532, DE-A 103 32 758 and DE-A 19 924 533). A removal of acrylic acidmay also be undertaken as described in EP-A 98 22 87, EP-A 98 22 89,DE-A 103 36 386, DE-A 101 15 277, DE-A 196 06 877, DE-A 197 40 252, DE-A196 27 847, EP-A 92 04 08, EP-A 10 68 174, EP-A 10 66 239, EP-A 10 66240, WO 00/53560, WO 00/53561, DE-A 100 53 086 and EP-A 98 22 88.Favorable ways of removal are also the processes described in thedocuments WO 2004/063138, WO 2004/035514, DE-A 102 43 625 and DE-A 10235 847.

The further removal of the acrylic acid from the liquid (condensed)phases which comprise the acrylic acid target product and are obtainedin the basic separation described is undertaken in the processes of theknown prior art, depending on the other by-products, in particular thosedependent upon the specific catalysts used for the partial oxidation andother selected partial oxidation conditions, comprised in addition toacrylic acid, by a wide variety of combinations of extractive,desorptive, rectificative, azeotropically distillative and/orcrystallizative processes up to the desired degree of purity of theacrylic acid. Particularly high purity demands are made on the acrylicacid when it is to be used to prepare water-superabsorbent polymers(polyacrylic acids or alkali metal salts thereof), since such polymersfind use in particular in the hygiene sector, where medical standardsapply. The aim of the predominant number of acrylic acid preparationprocesses is therefore a very economically viable route to such glacialacrylic acid suitable for superabsorbents.

A characteristic feature of the conventional acrylic acid preparationroute described is that it includes by-product formation of methacrylicacid at most in amounts which are analytically undetectable oranalytically insignificant, which can be attributed primarily to thehigh degree of purity of the crude propylene used. Thus, none of thefollowing prior art documents even mentions methacrylic acid as apossible secondary component of acrylic acid, even though the majorityof these documents comprise highly detailed secondary componentanalyses: WO 98/01414, WO 01/92197, EP-A 648 732, EP-A 1305097, EP-A 1484 308, EP-A 14 84 309, US-A 2004/0242826, DE-A 103 36 386, WO02/055469, WO 03/078378, WO 01/77056, WO 03/041833, DE-A 196 06 877,EP-A 792 867, EP-A 920 408, EP-A 10 15 411, EP-A 10 15 410, DE-A 198 38845, WO 03/041833, WO 02/090310, DE-A 101 22 787, WO 03/041832, EP-A 1068 174, EP-A 10 66 239, EP-A 11 63 201, EP-A 11 59 249, EP-A 11 89 861,EP-A 12 52 129, WO 01/77056, DE-A 102 35 847, DE-A 102 43 625 and WO2004/035514. The same statement applies to the documents referred to asthe particular state of the art in the aforementioned documents.

On the other hand, methacrylic acid, as would then be formed as asecondary component companion in a preparation of acrylic acid, would bea particularly unpleasant companion to acrylic acid which would notremain unmentioned in the prior art. This would be the case inparticular because the tendency of methacrylic acid to free-radicalpolymerization, owing to the positive inductive effect of the methylgroup which distinguishes it from acrylic acid, is significantly reducedin comparison to the same tendency of acrylic acid.

In other words, when acrylic acid which comprises methacrylic acid evenonly in traces is used to prepare water-superabsorbent free-radicallypolymerized polymers, it has to be assumed that the methacrylic acid isnot sufficiently polymerized under the particular polymerizationconditions selected and remains in the polymer formed as a vinylicallyunsaturated compound, which is problematic in applications in thehygiene sector. Presence of methacrylic acid can also adversely effectthe polymer quality (for example molecular weight distribution, degreeof crosslinking, etc.).

In the search for a more economically viable propylene source which canbe used for a heterogeneously catalyzed partial oxidation to acrylicacid, it has also already been proposed to start from crude propane andconvert it, in a reaction stage preceding the propylene partialoxidation, by homogeneous and/or heterogeneously catalyzedoxydehydrogenation and/or heterogeneously catalyzed dehydrogenation,partially to propylene, and to use the latter for the relevant partialoxidation without removing it in a costly and inconvenient manner fromunconverted propane (cf., for example, WO 03/076370, WO 01/96271, EP-A117 146, WO 03/011804, U.S. Pat. No. 3,161,670, DE-A 33 13 573, WO01/96270 and the prior art referred to in these documents). According toDE-A 102 46 119 and DE-A 102 45 585, the procedure should be such thatsuitable separation steps ensure that the resulting starting reactiongas mixture for the propylene partial oxidation comprises a minimumlevel of C₄ hydrocarbons as undesired impurities which impair thecatalyst performance. A disadvantage of such a procedure is that theaforementioned separation operations are costly and inconvenient, andare economically in some cases prohibitive for crude propylene envisagedmerely as an acrylic acid raw material, or achieve only limitedseparating action when they are employed more economically.

At the same time, crude propanes which comprise saturated or unsaturatedC₄ hydrocarbons to a significant extent are available particularlyinexpensively on the market, whether they occur as poorly utilizablesecondary streams on the route to the preparation of ultrapure crudepropane or a costly and inconvenient C₃/C₄ hydrocarbon separation hasbeen dispensed with fully in the course of their generation.

In the individual case such a favorable raw material price is thencapable of economically overcompensating an accompanying reduction inthe catalyst performance in a downstream heterogeneously catalyzedacrylic acid preparation by partial oxidation, or a prematurerequirement for a catalyst change.

A remaining disadvantage of a procedure for preparing acrylic acid whichmight otherwise be attractive as described is that it is accompanied,depending on catalysts used for the heterogeneously catalyzed partialoxidation of propylene afflicted with corresponding C₄ hydrocarboncontents to prepare acrylic acid, by methacrylic acid secondarycomponent formation owing to a partial oxidation of the C₄ hydrocarbons(e.g. isobutene and isobutane) proceeding in parallel to the mainpropylene partial oxidation, with the disadvantages described (cf. DE-A102 19 686, DE-A 33 13 573 and EP-A 297 445). The same disadvantages mayaccrue depending on the catalyst and reaction conditions used whenacrylic acid is obtained by partial direct oxidation of propanecomprising C₄ hydrocarbons as impurities, as is detailed, for example,in EP-A 608 838, DE-A 198 35 247, and also the documents DE-A 102 45 585and DE-A 102 46 119. Another possible C₃ precursor which may be burdenedwith C₄ hydrocarbons or their oxidative derivatives for aheterogeneously catalyzed preparation by partial oxidation of acrylicacid is acrolein (cf. EP-A 700 893 and EP-A 700 714).

SUMMARY OF THE INVENTION

It was therefore an object of the present invention to provide a veryefficient process for removing methacrylic acid from liquid phasecomprising acrylic acid as a main constituent and target product, andmethacrylic acid as a secondary component.

Accordingly, a process has been found for removing methacrylic acid fromliquid phase P comprising acrylic acid as a main constituent and targetproduct, and methacrylic acid as a secondary component, which compriseseffecting the removal by crystallization, the acrylic acid accumulatingin the crystals formed and the methacrylic acid in the remaining motherliquor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A procedure as described above is efficient only when there issubstantially no incorporation of methacrylic acid into the crystal inthe course of the formation of an acrylic acid crystal. This isgenerally the case when the depletion coefficient A^(MAS) associatedwith the crystallization is ≧15. The depletion coefficient A isunderstood generally to be the ratio of impurity remaining in the motherliquor to impurity remaining in the crystals (in each case expressed as% by weight based on the total amount of mother liquor or the totalamount of crystals; for example, mother liquor and crystals can beseparated substantially fully from one another by centrifugation or bycentrifugation and/or washing, and can be determined by subsequentanalysis A; to this end, a mother liquor removal to an extent of morethan 90% by weight, preferably to an extent of more than 95% by weight,or 97 or 98% by weight, or 99% by weight, of its total amount isgenerally sufficient).

In the case of acetic acid (A^(ES)) and propionic acid (A^(PS)) asacrylic acid impurities, the depletion coefficient is typically atvalues of ≦10. In other words, they are also incorporated into theacrylic acid crystals and can only be extracted from these crystals withdifficulty, for example by suitable washing. In other words, acrystallizative removal of these two impurities from acrylic acidgenerally entails the use of low-efficiency and capital-intensivemultistage crystallization processes, as are recommended, for example,in EP-A 616 998 in the form of a multistage combination of dynamic andstatic crystallization and which, in addition to the multistage process,require at least one dynamic and at least one static crystallizer. Atbest under particular boundary conditions of crystallization (cf. inparticular WO 03/078378 and WO 01/77056), acrylic acid crystal forms areformed, from which acetic acid and propionic acid can be removedcomparatively readily by subsequent washing with pure acrylic acid melt.

The greater A^(MAS) is, the more attractive a crystallizative removal ofmethacrylic acid from acrylic acid is.

As a result of detailed investigations, it has been found that,surprisingly, in the crystallizative removal of methacrylic acid fromliquid phase comprising acrylic acid as a main constituent and targetproduct, and methacrylic acid as a secondary component, evenunaccompanied centrifugal mother liquor/crystals removal is generallyaccompanied by depletion coefficients A^(MAS) of up to 30 (in words:thirty) and more (in the case of a wash column removal, this correspondsto values of A^(MAS) of at least ≧100, in favorable cases of ≧1000),while, in the converse case of a crystallizative removal of acrylic acidfrom liquid phase comprising methacrylic acid as the main constituentand target product, and acrylic acid as the secondary component, theacrylic acid depletion coefficients A^(AS) are generally ≦15. Thesefindings are unexpected. They are unexpected in particular because pureacrylic acid (m.p.=14° C.) and pure methacrylic acid (m.p.=15° C.) havevirtually identical melting points (at pressure 1 bar) (cf., forexample, Römpps Chemie-Lexikon). In contrast, their boiling behavior atstandard pressure (acrylic acid b.p.=141° C., methacrylic acid b.p.=161°C.) differs substantially more markedly and suggests, for example, arectificative removal of methacrylic acid. The aforementioned physicalfacts also demonstrate that, in the crystallizative purifications ofacrylic acid which are generally carried out in the prior art, saidacrylic acid having already been prepurified by means of other thermalseparation processes (cf., for example, EP-A 616 998), acrylic acids arefurther purified which are entirely free of methacrylic acid. In thisdocument, free of methacrylic acid means that methacrylic acid can nolonger be detected by gas chromatography.

With the aforementioned experimental findings, the inventive procedureopens up the possibility, on the route to the preparation of glacialacrylic acid suitable for superabsorbents, of removing the methacrylicacid impurities which obstruct such a use in a single separation step,in a single crystallization stage in a satisfactory manner.

The phrase “liquid phase P (or gas mixture or product gas mixture)comprising acrylic acid as a main constituent and target product, andmethacrylic acid as a secondary component” is intended in this documentmerely to mean that the liquid phase P (or the gas mixture or productgas mixture) comprises acrylic acid and methacrylic acid in a molarratio V of acrylic acid to methacrylic acid of at least 3:2. V in theprocess according to the invention may of course also be at least 2:1,or at least 3:1, or at least 4:1, or at least 5:1, or at least 6:1, orat least 7:1, or at least 8:1, or at least 9:1, or at least 10:1. Theprocess according to the invention is also appropriate when V is atleast 15:1, or at least 20:1, or at least 25:1, or at least 30:1, or atleast 35:1, or at least 40:1, or at least 45:1, or at least 50:1, or atleast 60:1, or at least 70:1, or at least 80:1, or at least 90:1, or atleast 100:1.

In many cases relevant to the application, V will be at least 200:1, orat least 300:1, or at least 400:1, or at least 500:1, or at least 600:1,or at least 700:1, or at least 800:1, or at least 900:1, or at least1000:1.

However, the process according to the invention is also important when Vis at least 2000:1, or at least 3000:1, or at least 4000:1, or at least5000:1, or at least 6000:1, or at least 7000:1, or at least 8000:1, orat least 9000:1, or at least 10 000:1. V in the process according to theinvention may of course also be at least 20 000:1, or at least 30 000:1,or at least 40 000:1, or at least 50 000:1, or at least 60 000:1, or atleast 70 000:1, or at least 80 000:1, or at least 90 000:1, or at least100 000:1.

In other words, the process according to the invention is stillsignificant especially in the context of an industrial-scale preparationof acrylic acid when the liquid phase P (or the gas mixture or productgas mixture), based on its content of acrylic acid, comprises only 10ppm by weight of methacrylic acid.

In other words, V in the process according to the invention may, forexample, be from 3:2 to 100 000:1, or from 2:1 to 70 000:1, or from 3:1to 50 000:1, or from 4:1 to 30 000:1, or from 5:1 to 10 000:1, or from6:1 to 8000:1, or from 7:1 to 6000:1, or from 8:1 to 5000:1, or from 9:1to 2000:1, or from 10:1 to 1000:1, or from 20:1 to 800:1, or from 30:1to 600:1, or from 40:1 to 400:1, or from 50:1 to 300:1, or from 60:1 to200:1, or from 70:1 to 100:1.

The above remarks are relevant especially when the liquid phase Pcomprises at least 10% by weight of acrylic acid, or at least 20% byweight of acrylic acid, or at least 30% by weight of acrylic acid, or atleast 40% by weight of acrylic acid, or at least 50% by weight ofacrylic acid, or at least 60% (or at least 65%) by weight of acrylicacid, or at least 70% by weight of acrylic acid, or at least 80% byweight of acrylic acid, or at least 90% by weight of acrylic acid, or atleast 93% by weight of acrylic acid, or at least 94% by weight ofacrylic acid, or at least 95% by weight of acrylic acid, or at least 96%by weight of acrylic acid, or at least 97% by weight of acrylic acid, orat least 98% by weight of acrylic acid, or at least 99% by weight ofacrylic acid, or at least 99.5% by weight of acrylic acid, or at least99.7% by weight of acrylic acid, or at least 99.9% by weight of acrylicacid, or at least 99.95% by weight of acrylic acid, or even more acrylicacid.

Based on the amounts of acrylic acid comprised in the aforementionedliquid phases P, their methacrylic acid content may, in a manner typicalof the process according to the invention, be from 0.001 to 15% byweight, or from 0.01 to 15% by weight, or from 0.02% by weight to 10% byweight, or from 0.03% by weight to 7% by weight, or from 0.04% by weightto 5% by weight, or from 0.05% by weight to 3% by weight, or from 0.07%by weight to 2% by weight, or from 0.09% by weight to 1.5% by weight, orfrom 0.1 or 0.15 to 1 or to 0.5% by weight.

Whether acrylic acid crystals separate out or not in the course ofcooling of liquid phases P having such a composition depends in theindividual case upon the overall composition of the liquid phase P.According to the teaching of WO 03/078378, this may be the case even forliquid phases P which comprise from 0.5 to 90% by weight, or from 7 to50% by weight, or from 10 to 25% by weight, or else from 10 to 85% byweight, or from 15 to 80% by weight, or from 25 to 75% by weight, ofwater.

EP-A 002 612 discloses that the eutectic of water-acrylic acid iseliminated, for example, by addition of salts to give aqueous acrylicacid solutions and thus that a crystallization of acrylic acid can bebrought about even at relatively low acrylic acid contents (thisauxiliary measure may also be employed in the process according to theinvention).

In a similar manner, WO 99/06348 recommends the addition of polarorganic substances before a crystallizative acrylic acid removal fromaqueous phases P (this auxiliary measure may also be employed in theprocess according to the invention).

DE-A 198 38 845 teaches that acrylic acid is generally separated in thecourse of cooling from liquid phases P which comprise acrylic acid andan organic solvent having a higher boiling point than acrylic acid understandard conditions when the liquid phase P comprises from >60 to <99.9%by weight of acrylic acid, from 0.1 to 40% by weight of high-boilingorganic solvent and from >0 to 35% by weight of secondary componentsobtained in the catalytic acrylic acid preparation in the gas phase. Theliquid phases P of DE-A 198 38 845, which forms an integral part of thisdocument, are also useful for the process according to the inventionprovided that they have the methacrylic acid impurity required inaccordance with the invention.

According to the above, it can be assumed that acrylic acid regularlycrystallizes out at least in the course of cooling of liquid phases Pwhich comprise ≧65% by weight of acrylic acid and are to be treated inaccordance with the invention.

The process according to the invention can therefore be appliedadvantageously in particular to liquid phases P contaminated withmethacrylic acid when they comprise from 65 to 99.5% by weight, or from70 to 99.5% by weight, or from 80 to 99.5% by weight, or from 85 to 99%by weight, or from 90 to 98% by weight, or from 93 to 97% by weight, ofacrylic acid. This is the case in particular when the methacrylic acidcontents, based on acrylic acid comprised, are simultaneously from 0.01to 15% by weight, or from 0.02 to 10% by weight, or from 0.03 to 7% byweight, or from 0.04 to 5% by weight, or from 0.05 to 3% by weight, orfrom 0.07 to 2% by weight, or from 0.09 to 1.5% by weight, or from 0.1or 0.15 to 1 or to 0.5% by weight.

Otherwise, liquid phases P to be treated in accordance with theinvention may, in a manner known per se, be obtained from (stem from)product gas mixtures, comprising acrylic acid as a main constituent andmethacrylic acid as a secondary component, of heterogeneously catalyzedpartial oxidations of C₃ precursors of acrylic acid contaminated, forexample, in the manner described (propane, propylene and/or acrolein;possible C₃ precursors also include propionic acid, propanol and/orpropionaldehyde; in this case, the partial oxidation is aheterogeneously catalyzed oxydehydrogenation) as already described inthe prior art. The remaining composition of these product gas mixtureswill substantially be that as known from the known oxidative acrylicacid preparations in the gas phase. In addition, the inventivecrystallization process may be practiced in the same way and beintegrated in the same way into the overall process for removing(glacial) acrylic acid from the product mixture, as taught in particularby the following prior art documents, all of which are an integral partof this document (typical methacrylic acid contents of such product gasmixtures are, based on the amount of acrylic acid comprised therein, forexample from 0.01 to 15% by weight, or to 10% by weight, or to 5% byweight, frequently from 0.02 to 4% by weight, in many cases from 0.03 to3% by weight, often from 0.04 to 2% by weight, but also from 0.05 to 1%by weight, or from 0.07 to 0.75% by weight, or from 0.1 to 0.5% byweight, or from 0.2 to 0.4% by weight): WO 02/055469, WO 03/078378, WO01/77056, WO 03/041833, DE-A 196 06 877, DE-A 103 36 386, WO 98/01414,WO 01/77056, EP-A 14 84 308, EP-A 14 84 309, US-A 2004/0242826, DE-A 10243 625, DE-A 196 06 877, EP-A 792 867, EP-A 10 15 410, EP-A 920 408,EP-A 11 89 861, EP-A 10 15 411, EP-A 10 68 174, WO 2004/035514, EP-A 1066 293, EP-A 11 163 201, EP-A 1159 249; WO 02/090310, DE-A 101 22 787,WO 03/041832, DE-A 102 35 847, EP-A 12 52 129, EP-A 616 998, EP-A 13 88533, EP-A 11 25 912 and EP-A 11 16 709.

The process according to the invention is of very particularsignificance when the liquid phase P which comprises acrylic acid as amain constituent and target product, and methacrylic acid as a secondarycomponent, and is to be treated in accordance with the invention isobtained from the product gas mixture of a heterogeneously catalyzedpartial gas phase oxidation of at least one C₃ precursor of acrylic acidusing at least one indistinct separation process. This is the caseespecially when the mother liquor remaining in the subsequentcrystallizative removal of the remaining methacrylic acid is recycled atleast partly into at least one of the indistinct separation processes.

The basic structure of such a combined use of indistinct separationprocesses and the distinct separation process of crystallization istaught, for example, by DE-A 196 06 877, EP-A 792 867, and also EP-A 1484 308, EP-A 14 84 309, EP-A 11 16 709 and in particular EP-A 10 15 410.

An indistinct separation process is defined as a separation process inwhich the composition of the phase which is formed when the separationprocess is employed and comprises accumulated target product isdependent markedly upon the composition of the mixture to be separated,while the inventive crystallizative treatment is a distinct separationprocess to the extent that the composition of the acrylic acid crystalswhich form is substantially independent (ideally there is completeindependence) of the composition of the liquid phase P.

In the case of such a combination of a distinct and an indistinctseparation process, the process according to the invention is ofincreased significance in as far as, in the continuous operation of sucha procedure, the methacrylic acid accumulates in the liquid phase P tobe treated in accordance with the invention as a result of the motherliquor recycling, since the mother liquor comprises the methacrylic acidin accumulated form. In other words, even comparatively smallmethacrylic acid contents in the product gas mixture of the gas phaseoxidation can thus grow to become a serious problem. Increasedmethacrylic acid contents may also be comprised in liquid phases P, forexample, when mother liquors obtained in the process according to theinvention are crystallized further for the purpose of increasing theyield, or when methacrylic acid-comprising secondary streams obtained inthe indistinct separation process are treated in accordance with theinvention to increase the yield.

This means that, in the case of depletion coefficients A^(MAS)<15, sucha procedure would be extremely inefficient. The efficiency required onthe industrial scale is gained only by, surprisingly in accordance withthe invention, an A^(MAS) of >15 having been found.

In general, the at least one indistinct separation process employed toobtain the liquid phase P to be treated in accordance with the inventionfrom the product gas mixture of a heterogeneously catalyzed partial gasphase oxidation of at least one C₃ precursor of acrylic acid will be adistillation, rectification, absorption, adsorption, extraction,desorption, stripping, distraction, (partial) condensation, fractionalcondensation, a membrane separation process such as apervaporation/vapor permeation or a combination of such processes.

Particularly frequently, a distillation, rectification, absorption,extraction, partial condensation, fractional condensation, desorption,stripping and/or distraction will be employed. Frequently, the liquidphase P to be treated in accordance with the invention will be obtainedby employing the aforementioned processes repeatedly.

In the simplest case, the liquid phase P to be treated in accordancewith the invention may be the absorbent and/or partial condensate and/orcondensate obtained by fractionation, of an absorptive and/orcondensative removal of acrylic acid from the product gas mixture of aheterogeneously catalyzed partial gas phase oxidation of at least one ofthe C₃ precursors listed in this document. In that case, preference isgiven in accordance with the invention to mother liquor being recycledinto the absorption and/or condensation.

In an appropriate manner, a combination of indistinct and distinctseparation to be employed as described has an outlet for at least onestream comprising accumulated methacrylic acid. Advantageously inaccordance with the invention, this is on the side of the indistinctseparation process. In general, the bottoms liquid of a separationcolumn will be used as such an outlet, from which column the liquidphase P itself or the stream to be converted later to the liquid phase Pis withdrawn, for example, via side withdrawal and/or via topwithdrawal.

However, a methacrylic acid outlet may be disposed also or only on theside of the inventive removal, i.e. on the crystallizative side. In thiscase, the outlet will consist of mother liquor comprising accumulatedmethacrylic acid.

When the inventive removal is performed, for example, by means of acombination of dynamic and static crystallization according to EP-A616998, the mother liquor outlet comprising accumulated methacrylic acidwill be disposed in the region of the static crystallization. The latteris the case in particular when no mother liquor recycling into anindistinct separation is carried out when the process according to theinvention is employed.

The process according to the invention is favorable not least when theliquid phase P to be treated in accordance with the invention (forexample by means of one of the above-described procedures) stems from aproduct gas mixture of a partial oxidation of at least one C₃ precursorof acrylic acid, comprises the methacrylic acid in the already statedcontents based on the acrylic acid content of the product gas mixture,and otherwise comprises:

from 1 to 30% by volume of acrylic acid,

from ≧0 or 0.005 to 10% by volume of propylene,

from ≧0 or 0.001 to 2% by volume of acrolein,

from ≧0 or 0.001 to 2% by volume of methacrolein,

from ≧0 or 0.005 to 10% by volume of molecular oxygen,

from ≧0 or 0.005 to 3% by volume of acetic acid,

from ≧0 or 0.001 to 2% by volume of propionic acid,

from ≧0 or 0.001 to 2% by volume of formaldehyde,

from ≧0 or 0.0001 to 2% by volume of other aldehydes,

and from 10 to 98% by volume or from 50 to 98% by volume of (inert)diluent gases.

The diluent gases may comprise:

from ≧0 or 0.005 to 90% by volume of saturated C₁- to C₆-hydrocarbons(especially propane, methane and/or ethane),

from ≧0 or 0.05 to 30% by volume of steam,

from ≧0 or 0.05 to 15% by volume of carbon oxides (CO and/or CO₂),

and from ≧0 or 1 to 90% by volume of molecular nitrogen.

The partial oxidation product gas mixture may in particular stem from apartial oxidation as described in the documents DE-A 10 2004 032 129 andtheir equivalent foreign patents, DE-A 10245585, WO 03/076370, WO01/96271, EP-A 117146, WO 03/011804, U.S. Pat. No. 3,161,670, DE-A3313573, DE-A 10316039 and WO 01/96270, starting from propylene and/orpropane, and has as the propylene source, if appropriate, a propanedehydrogenation and/or oxydehydrogenation (heterogeneously catalyzed ifappropriate) as a preceding reaction stage.

A representative example (i.e. just one possible example) of such aproduct gas mixture composition is a product gas mixture which comprises(all secondary components listed would be removed satisfactorily whenthe process according to the invention is employed if they had been aconstituent of the liquid phase P):

% by vol. nitrogen 51.54 oxygen 2.3 propane 29.20 propene 0.110 methane0 ethane 0.077 n-butane 0.101 isobutane 0.236 n-butenes 0 isobutene0.001 1,3-butadiene 0.009 hydrogen 0.05 carbon monoxide 0.596 carbondioxide 1.72 water 8.21 acrolein 0.09 acrylic acid 5.28 acetic acid0.240 propionic acid 0.002 formic acid 0.019 formaldehyde 0.198benzaldehyde 0.005 maleic anhydride 0.047 methacrolein 0.020 methacrylicacid 0.011 and ethene 0.032.

Advantageously in accordance with the invention, the liquid phase P tobe treated in accordance with the invention will be obtained from theaforementioned product gas mixtures of the C₃ acrylic acid precursorpartial oxidation by condensing acrylic acid out of the product gasmixture. Advantageously in accordance with the invention, the condensateobtained directly forms the liquid phase P. Advantageously, thecondensation of acrylic acid out of the product gas mixture (which hasbeen cooled beforehand if appropriate) is effected as a fractionalcondensation (on which is, if appropriate, additionally superimposed anabsorption with water and/or aqueous solution in order to reduce acrylicacid losses) as described in detail, for example, in the documents EP-A1015410, WO 2004/035514, DE-A 10243625, EP-A 1015411, DE-A 10235847,EP-A 1159249, EP-A 1163201, EP-A 1066239 and EP-A 920408.

In this condensation, the product gas mixture is appropriately, ifappropriate after preceding direct and/or indirect cooling (for examplewith a quenched liquid according to EP-A 1066239, or according to EP-A1163201), fractionally condensed in a separating column havingseparating internals, ascending into itself with side draw removal ofcrude acrylic acid (which preferably forms the liquid phase P to betreated in accordance with the invention; if appropriate the crudeacrylic acid is also treated rectificatively and/or distillatively toobtain the liquid phase P).

Liquid phase P obtained condensatively (and if appropriate additionallyrectificatively) in this way can then be treated crystallizatively inaccordance with the invention. Mother liquor comprising accumulatedmethacrylic acid which is formed will, according to the model, forexample, of EP-A 920408 or WO 2004/035514, be recycled at least partly,preferably fully, into the condensation of acrylic acid out of theproduct gas mixture. The methacrylic acid outlet will be located belowthe side draw of the crude acrylic acid.

Liquid phase P which has been obtained in this way by partial or totalcondensation and/or superimposed absorption with water or aqueoussolution and also, if appropriate, rectificative aftertreatment, and isto be treated in accordance with the invention, may, in addition to thecontents of methacrylic acid based on acrylic acid comprised which havealready been indicated (and are essential to the invention), comprise:

from 65, or 75, or 85 to 99.5% by weight of acrylic acid,

from ≧0, generally from 0.1 to 40% by weight of water,

from ≧0, in general from 0.001 to 5% by weight of acrolein,

from ≧0, in general from 0.001 to 10% by weight of methacrolein,

from ≧0, in general from 0.01 to 5% by weight of acetic acid,

from ≧0, in general from 0.01 to 5% by weight of propionic acid,

from ≧0, in general from 0.001 to 5% by weight of formaldehyde,

from ≧0, in general from 0.001 to 5% by weight of further aldehydes (peraldehyde), and

from ≧0, in general from 0.01 to 5% by weight of maleic acid.

Advantageously, the aforementioned contents of the liquid phase P willbe:

from 93 to 98% by weight of acrylic acid,

from 1 to 5% by weight of water,

from 0.001 to 3% by weight of acrolein,

from 0.001 to 3% by weight of methacrolein,

from 0.1 to 3% by weight of acetic acid,

from 0.01 to 3% by weight of propionic acid,

from 0.001 to 3% by weight of formaldehyde,

from 0.001 to 3% by weight of further aldehydes (per aldehyde) and

from 0.01 to 3% by weight of maleic acid.

Following the specifications of WO 02/055469 and WO 03/078378, it mayalso comprise up to 3% by weight of protoanemonin.

The inventive crystallizative treatment of the liquid phase P,especially of a liquid phase P obtained condensatively and/orabsorptively and/or rectificatively in the aforementioned manner, is inprinciple subject to no restriction, including the process for removingthe mother liquor from the crystals (all processes detailed in the priorart mentioned can be employed).

In other words, it may be carried out in one or more stages,continuously or batchwise. In particular, it may also be carried out asa fractional crystallization. Typically, in a fractionalcrystallization, all stages which generate acrylic acid crystals whichare purer (especially freer from methacrylic acid) than the liquid phaseP supplied are known as purification stages and all other stagesstripping stages. Appropriately, multistage processes are operated bythe countercurrent principle, in which, after the crystallization ineach stage, the crystals are removed from the mother liquor and thesecrystals of the particular stage are fed with the next highest degree ofpurity, while the crystallization residue of the particular stage is fedwith the next lowest degree of purity.

In general, the temperature of the liquid phase P during the processaccording to the invention is between −25° C. and +14° C., in particularbetween 12° C. and −5° C.

For example, the process according to the invention may be performed asa layer crystallization (cf. DE-A 2606364, EP-A 616998, EP-A 648520 andEP-A 776875). In this crystallization, the crystals are frozen out inthe form of continuous, firmly adhering layers. The deposited crystalsare separated from the remaining residual melt (the mother liquor) byvirtue of the residual melt simply flowing off. In principle, adistinction is drawn between “static” and “dynamic” layercrystallization processes. A characteristic feature of dynamic layercrystallization of liquid phases P is forced convection of the liquidphase P. This can be effected by pumped circulation of the liquid phasethrough tubes with full flow-through, by introduction of the liquidphase P as a trickle film (for example according to EP-A 616998) or byintroduction of inert gas into a liquid phase P or by pulsation.

In the static processes, the liquid phase P is at rest (for example intube bundle or plate heat exchangers) and deposits in layers as resultof slow temperature reduction on the secondary side. Afterward, theresidual melt (mother liquor) is discharged, more highly contaminatedfractions are sweated off from the crystal layer by slow temperatureincrease and the pure product is subsequently melted off (cf. WO01/77056).

According to the invention, the process according to the invention, inthe case of all liquid phases P described in this document, will,however, preferably be performed according to the teaching of WO01/77056, WO 02/055469 and WO 03/078378 as a suspension crystallization.

In general, a crystal suspension comprising suspended acrylic acidcrystals is obtained by cooling the liquid phase P, the acrylic acidcrystals having a lower methacrylic acid content and the remainingresidual melt (mother liquor) a higher methacrylic acid content(relatively, based on the particular total amount) than the liquid phaseP to be purified. The acrylic acid crystals may grow directly insuspension and/or be deposited as a layer on a cooled wall from whichthey are subsequently scratched off and resuspended in the residual melt(mother liquor).

All suspension crystallizers and suspension crystallization processesdetailed in WO 01/77056, WO 02/055469, and WO 03/078378 are useful inaccordance with the invention. In general, the acrylic acid crystalsuspension generated has a solids content of from 20 to 40% by weight.

In addition, all processes specified in the aforementioned WOpublications are suitable for the separation of suspension crystalswhich have formed and mother liquor which remains (for examplemechanical separation processes such as centrifugation). Preference isgiven in accordance with the invention to separating in a wash column.This is preferably a wash column with forced transport of the depositedacrylic acid crystals. The crystal volume fraction in the crystal bedgenerally attains values of >0.5. In general, the wash column isoperated at values of from 0.6 to 0.75. The wash liquid used isadvantageously the melt of acrylic acid crystals purified (removed)beforehand in the wash column. The washing is normally effected incountercurrent. The process according to the invention thus inparticular comprises processes which comprise the following processsteps:

-   a) crystallization of acrylic acid out of a liquid phase P;-   b) separation of the acrylic acid crystals from the remaining mother    liquor (residual melt, liquid residual phase);-   c) at least partial melting of the removed acrylic acid crystals and-   d) at least partial recycling of the molten acrylic acid crystals to    step b) and/or to step a).

Preference is given to effecting step b) by countercurrent washing withacrylic acid crystals which have been removed beforehand, melted andrecycled into step b).

Advantageously in accordance with the invention, the liquid phase Pcomprises water when the process according to the invention is employed,since formation of acrylic acid crystals in the presence of water,according to the teaching of WO 01/77056 and WO 03/078378, causes aparticularly favorable crystal form for the subsequent separation of thecrystals from the remaining mother liquor. This is especially true whenthe crystallization is performed as a suspension crystallization, andeven more true when the subsequent mother liquor removal is performed ina wash column, and even more true when the wash liquid used is the meltof acrylic acid crystals which have already been purified in the washcolumn.

In other words, the process according to the invention comprises inparticular processes in which the liquid phase P to be purified isconverted under the action of cold conditions to a crystal suspensionconsisting of acrylic acid crystals and liquid residual phase (residualmelt), the proportion by weight of methacrylic acid in the acrylic acidcrystals being smaller and the proportion by weight of the liquidresidual phase (the mother liquor) of methacrylic acid being greaterthan the proportion by weight of methacrylic acid in the liquid phase P,a portion of the remaining mother liquor is removed mechanically ifappropriate from the crystal suspension, and the acrylic acid crystalsare freed in a wash column of remaining mother liquor, with the provisothat

-   a) the liquid phase P, based on the acrylic acid comprised therein,    comprises from 0.20 to 30% by weight, frequently up to 20% by    weight, often up to 10% by weight, of water, and-   b) the wash liquid used is the melt of acrylic acid crystals    purified in the wash column.

In particular, the process according to the invention comprises theaforementioned processes, the liquid phase P comprising ≧80% by weightof acrylic acid, or ≧90% by weight of acrylic acid or ≧95% by weight ofacrylic acid. The molar ratio V in the liquid phase P to be purified mayin each case have all of the values specified in this document.

Moreover, it is advantageous in accordance with the invention when thewater content of the liquid phase P in the above-described procedures(or quite generally when the process according to the invention isemployed), based on acrylic acid comprised in the liquid phase P, isfrom 0.2 or 0.4 to 8, or to 10, or to 20, or to 30% by weight, or from0.6 to 5% by weight, or from 0.60 to 3% by weight.

The process according to the invention can of course also be applied toall crude acrylic acids of WO 98/01414, provided that they additionallycomprise methacrylic acid as a secondary component.

All of the aforementioned applies in particular when the wash column isa wash column having forced transport of the acrylic acid crystals, inparticular when it is a hydraulic or a mechanical wash column accordingto WO 01/77056 and is operated as detailed therein.

All of the aforementioned is true in particular when the wash column isdesigned and operated according to the teachings of WO 03/041832 and ofWO 03/041833.

The process according to the invention thus permits, with the sequenceof partial oxidation of at least one C₃ precursor, fractional acrylicacid condensation from the product gas mixture of the partial oxidation,suspension crystallization of the acrylic acid condensate withdrawn andremoval of the suspension crystals from remaining mother liquor in awash column using a pure crystal melt as the wash liquid, thepreparation of acrylic acid suitable for superabsorbents in a highlyefficient manner and using only one crystallization stage (such acrylicacid can of course also be used for all other uses addressed in WO02/055469 and WO 03/078378, even when the starting material for thepartial oxidation is a cheap C₃ precursor raw material source whichcauses the by-product formation of methacrylic acid).

Of course, all process steps detailed in this document are carried outwith polymerization inhibition. The procedure may be as described in theprior art listed. An excellent position among the entirety of theavailable acrylic acid process stabilizers is assumed bydibenzo-1,4-thiazine (PTZ), 4-hydroxy-2,2,6,6-tetramethylpiperidine1-oxyl (4-OH-TEMPO) and p-methoxyphenol (MEHQ), which either alone, orin pairs or as a three-substance mixture may be part of the liquid phaseP to be treated in accordance with the invention. Typically, the totalamount of polymerization inhibitors comprised in the liquid phase P,based on the total amount of acrylic acid and methacrylic acid comprisedtherein, is from 0.001 to 2% by weight.

Owing to undesired formation of acrylic acid oligomers (Michael adducts)in the liquid phase P when it is left alone, the process according tothe invention is employed as promptly as possible after generation ofthe liquid phase P.

In an advantageous manner in accordance with the invention, when theprocess according to the invention is employed, other C₄ (e.g. butene-1,butadiene, n-butane, etc.) subsequent partial oxidation productscomprised in the liquid phase P, for example methacrolein, butyricacids, butyraldehydes, etc. are also removed. Based on acrylic acid,they may be present in the same amounts as methacrylic acid in theliquid phase P (in particular in all liquid phases P detailed explicitlyin this document). The same applies to propionaldehyde and all C₅ and C₆subsequent partial oxidation products.

EXAMPLES AND COMPARATIVE EXAMPLE Comparative Example

430 g of a methacrylic acid were charged with a temperature of 25° C.into a stirred glass vessel (internal volume 1 l) with jacket cooling.The stirring was effected by means of a close-clearance helical stirrer.

The methacrylic acid comprised:

69.1% by weight of methacrylic acid,

18.2% by weight of water,

3.6% by weight of acetic acid and

0.5% by weight of acrylic acid.

It was obtained by heterogeneously catalyzed gas phase partial oxidationof methacrolein and subsequent fractional condensation of the productgas mixture. Addition of less than 200 ppm by weight of hydroquinone(based on the sum of acrylic acid and methacrylic acid) inhibited themethacrylic acid to polymerization.

At a cooling rate of 0.5 K/h, the temperature of the cooling liquidconducted within the jacket (water/glycol or water/ethanol mixture) wasreduced until the resulting crystal suspension (methacrylic acidcrystals suspended in residual melt) had a solids content of 16% byweight. A portion of the crystal suspension was then withdrawn andcentrifuged at 2000 rpm on a laboratory centrifuge in a sieve cupequipped with a polypropylene filter fabric for 30 seconds. Thiscentrifuged off the majority of mother liquor remaining. Subsequently,the mother liquor removal was completed by once again centrifuging asdescribed for 30 seconds and washing the crystals simultaneously with amethacrylic acid (methacrylic acid content >98% by weight, acrylic acidcontent <0.1% by weight, temperature of this wash methacrylic acid=25°C.) in a weight ratio of 1:1. Analysis of the remaining crystals and ofthe mother liquor centrifuged off beforehand gave the followingdepletion coefficients A for the different secondary componentscomprised in the methacrylic acid:

Secondary component A Water 41.7 Acetic acid 33.3 Acrylic acid 9.09

In contrast to water and acetic acid, acrylic acid is incorporated intothe deposited methacrylic acid crystals.

Examples

In each case approx. 1800 g of various acrylic acids were charged into astirred metal tank (internal volume 2 l, close-clearance helicalstirrer). They were polymerization-inhibited by addition of from 100 to200 ppm by weight of monomethyl ether of hydroquinone (MEHQ) and <100ppm by weight of phenothiazine (based on acrylic acid comprised).

At a cooling rate of 1 K/h, the temperature of the cooling liquid withinthe jacket was lowered until the resulting crystal suspension (acrylicacid crystals suspended in residual melt) had a solids content of ineach case approx. 18% by weight. A portion of the crystal suspension wasthen withdrawn and centrifuged at 2000 rpm on a laboratory centrifuge asin the comparative example for 180 seconds, and the remaining motherliquor was thus virtually fully centrifuged off. Analysis of theremaining crystals and of the mother liquor which had been centrifugedoff gave the depletion coefficients listed below for the differentcontaminated acrylic acids (liquid phases P to be treated in accordancewith the invention) for the secondary components comprised therein. Theacrylic acids were obtained starting from acrylic acid stemming from aheterogeneously catalyzed gas phase partial oxidation of propylenehaving a small C₄ fraction by addition of glacial methacrylic acidthereto. The starting acrylic acid comprised the following contents:

95.201% by weight of acrylic acid,

0.042% by weight of methacrylic acid,

0.604% by weight of benzaldehyde,

0.062% by weight of propionic acid,

0.687% by weight of furan-2-aldehyde,

0.663% by weight of acetic acid,

0.004% by weight of furan-3-aldehyde,

0.002% by weight of allyl acrylate,

0.009% by weight of acrolein and

2.20% by weight of water.

This acrylic acid was admixed once with 0.2% by weight of methacrylicacid (sample 1), once with 1% by weight of methacrylic acid (sample 2)and once with 5% by weight of methacrylic acid (sample 3) (based in eachcase on the weight of the starting acrylic acid) and the differentsamples were treated as described. In the same manner, the starting aciditself was treated. The depletion coefficients determined for thesecondary components comprised in the different samples and in thestarting acid were as follows:

secondary A (starting component A (sample 1) A (sample 2) A (sample 3)acid) methacrylic acid 31.3 45.5 30.3 no longer detectable in thecrystals benzaldehyde 31.3 45.4 31.2 31.2 propionic acid 3.9 4.1 3.853.86 furan-2-aldehyde 31.3 47.6 30.3 31.25 acetic acid 5.8 6.4 5.59 5.85

Remarkably, the ratios are reversed from the case of the comparativeexample. While acetic acid and propionic acid are incorporated here,this is the case neither for methacrylic acid nor the two aldehydes.

U.S. Provisional Patent Application Nos. 60/668,089, filed on Apr. 5,2005, and 60/656,877, filed on Mar. 1, 2005, are incorporated into thepresent application by literature reference. With regard to theabovementioned teachings, numerous alterations and deviations from thepresent invention are possible. It can therefore be assumed that theinvention, within the scope of the appended claims, can be performed ina different way from that specifically described herein.

1. A process for removing methacrylic acid from acrylic acid in a liquidphase P comprising acrylic acid and methacrylic acid with an acrylicacid content of at least 50% by weight and a molar ratio V of acrylicacid to methacrylic acid ranging from 3:2 to 100 000:1, which comprises:subjecting liquid phase P to crystallization thereby forming acrylicacid crystals and a mother liquor that contains methacrylic acid.
 2. Theprocess according to claim 1, wherein the liquid phase P comprises atleast 65% by weight of acrylic acid.
 3. The process according to claim1, wherein the liquid phase P, based on the amount of acrylic acidcomprised therein, comprises from 0.01 to 15% by weight of methacrylicacid.
 4. The process according to claim 1, wherein the liquid phase Pcomprises from 65 to 99.5% by weight of acrylic acid.
 5. The processaccording to claim 1, wherein the liquid phase P, based on the acrylicacid therein, comprises from 0.2 to 30% by weight of water.
 6. Theprocess according to claim 1, wherein the liquid phase P stems from theproduct gas mixture of a heterogeneously catalyzed gas phase partialoxidation of at least one C₃ precursor compound of acrylic acid.
 7. Theprocess according to claim 6, wherein the at least one C₃ precursorcompound is propane.
 8. The process according to claim 6, wherein the atleast one C₃ precursor compound is propylene.
 9. The process accordingto claim 6, wherein the at least one C₃ precursor compound is propyleneaccompanied by propane as an inert gas constituent.
 10. The processaccording to claim 9, wherein the propylene and the accompanying propaneare constituents of the product gas mixture obtained from the partialdehydrogenation and/or oxydehydrogenation of propane which precedes thegas phase partial oxidation.
 11. The process according to claim 1,wherein the liquid phase P stems from the product gas mixture of aheterogeneously catalyzed gas phase partial oxidation of at least one C₃precursor compound of acrylic acid and is obtained therefrom by at leastone indistinct separation process.
 12. The process according to claim11, wherein the at least one indistinct separation process comprises atleast one separation process which is an absorption process, a partialcondensation process, a fractional condensation process, a rectificationprocess, a stripping process, or a desorption process.
 13. The processaccording to claim 11, wherein said mother liquor comprising accumulatedmethacrylic acid is recycled into at least one of the at least oneindistinct separation process.
 14. The process according to claim 13,wherein said mother liquor comprising accumulated methacrylic acid isrecycled into a fractional condensation of the product gas mixture ofthe heterogeneously catalyzed gas phase partial oxidation of the atleast one C₃ precursor compound of acrylic acid.
 15. The processaccording to claim 1, wherein the crystallization of acrylic acid whichaccomplishes the removal of acrylic acid is done by suspensioncrystallization.
 16. The process according to claim 15, wherein theseparation of suspended crystals from mother liquor which forms isaccomplished by means of a wash column.
 17. The process according toclaim 16, wherein the wash liquid used is the melt of acrylic acidcrystals removed beforehand in the wash column.
 18. The processaccording to claim 1, which comprises the following process steps: a)crystallizing acrylic acid from liquid phase P, thereby forming crystalsof acrylic acid and a mother liquor; b) separating the acrylic acidcrystals from the mother liquor; c) at least partially melting theremoved acrylic acid crystals and d) at least partially recycling themolten acrylic acid crystals to step b) and/or to step a).
 19. Theprocess according to claim 1, which is followed by a process in whichacrylic acid crystals are melted and free-radically polymerized topolymers.
 20. The process according to claim 1, wherein liquid phase Pis comprised of: from 65 to 99.5% by weight of acrylic acid, from ≧0 to40% by weight of water, from ≧0 to 5% by weight of acrolein, from ≧0 to10% by weight of methacrolein, from ≧0 to 5% by weight of acetic acid,from ≧0 to 5% by weight of propionic acid, from ≧0 to 5% by weight offormaldehyde, from ≧0 to 5% by weight of other aldehydes (per aldehyde),and from ≧0 to 5% by weight of maleic acid.
 21. The process according toclaim 20, wherein liquid phase P is comprised of: from 93 to 98% byweight of acrylic acid, from 1 to 5% by weight of water, from 0.001 to3% by weight of acrolein, from 0.001 to 3% by weight of methacrolein,from 0.1 to 3% by weight of acetic acid, from 0.01 to 3% by weight ofpropionic acid, from 0.001 to 3% by weight of formaldehyde, from 0.001to 3% by weight of further aldehydes (per aldehyde) and from 0.01 to 3%by weight of maleic acid.
 22. The process according to claim 1, whereinmolar ratio V ranges from 2:1 to 70,000:1.
 23. The process according toclaim 1, wherein molar ratio V ranges from 3:1 to 50,000:1.
 24. Theprocess according to claim 1, wherein molar ratio V ranges from 4:1 to30,000:1.